1. Field of the Invention
The field of the invention relates generally to organic semiconductors and more particularly, to high performance organic semiconductor devices and a method for creating high performance organic semiconductor devices.
2. Background
Organic and polymer materials are currently being investigated for use as the active layer in various electronic devices. Organic materials provide advantages over silicon materials, including lower costs, compatibility with flexible substrates, and possibility of being printed or spin-coated to form electronic devices. Certain organic electronic devices have been already conceived and fabricated. For example, organic diodes, organic light-emitting diodes (OLEDs) and thin film transistors with an organic active layer are known. See, e.g., the devices described in: (1) Jie Liu, Tzung-Fang Guo and Yang Yang, xe2x80x9cEffects of thermal annealing on the performance of polymer light emitting diodesxe2x80x9d, J. Appl. Phys., 91, 1595-1600 (2002); (2) Jie Liu, Yijian Shi, and Yang Yang, xe2x80x9cSolvation-induced morphology effects on the performance of polymer-based photovoltaic devicesxe2x80x9d, Adv. Functional Materials, 11, pp 420-424, (2001); (3) Jie Liu, Yijian Shi, and Yang Yang, xe2x80x9cImproving the performance of polymer light-emitting diodes using polymer solid-solutionsxe2x80x9d, J. Appl. Phys. 89, 3668, (2001); (4) J. Liu, Y. Shi, and Y. Yang; xe2x80x9cSolvation induced morphological effects on the formation of polymer/metal contactsxe2x80x9d; J. Appl. Phys. 89, 3668 (2000); (5) Yang Yang, Shun-Chi Chang, Jayesh Bharathan, and Jie Liu, xe2x80x9cOrganic/polymeric electroluminescent devices processed by hybrid ink-jet printingxe2x80x9d, Journal of Materials Science: Materials in Electronics 11(2000) 89-96; (6) J. Liu, Y. Shi, and Y. Yang, Device performance and polymer morphology in polymer light-emitting diodes: the control of electric properties; J. Appl. Phys., 88, 605, (2000); (7) Y. Shi, J. Liu, and Y. Yang, Device performance and polymer morphology in polymer light-emitting diodes: the control of thin film morphology and device quantum efficiency; J. Appl. Phys., 87, 4254 (2000); (8) S. C. Chang, J. Bharathan, J. Liu, and Y. Yang, Multicolor organic light-emitting diodes processed by ink-jet printing, Adv. Mat. 11, 734, (1999); and (9) S. C. Chang, J. Bharathan, and Y. Yang; xe2x80x9cDual-color polymer LEDs processed by hybrid inkjet printing technologyxe2x80x9d, Appl. Phys. Lett., 73, 2561, (1998). The above references are incorporated herein by reference in their entireties.
As used herein, xe2x80x9corganic semiconductorxe2x80x9d refers to a material that contains a substantial amount of carbon in combination with other elements, or that comprises an allotrope of elemental carbon, excluding diamond, and exhibits a charge carrier mobility of at least 10xe2x88x923 cm2/V-s at room temperature (20xc2x0 C.). However, despite considerable research and development effort in organic semiconductor devices, these organic devices have not been widely commercialized yet, due in large part to their poor device characteristics relative to their silicon counterparts. While some organic light-emitting devices seem to perform similarly to inorganic light-emitting devices, the performance of other organic electronic devices, such as diodes, is poor as compared to their silicon counterparts. For example, organic diodes have a much lower frequency response and can handle a smaller current density than diodes made of silicon. These performance deficiencies are mainly due to the low carrier mobility and other characteristics of organic materials. Organic transistors, due to their low carrier mobility, can only handle low current densities and are not suitable for use as switches in high current density applications such as organic light-emitting diodes for displays. Nor are present day organic devices able to operate at high frequencies, such as 13 megahertz (MHz), 900 MHz and 2.4 gigahertz (GHz), where many of today""s silicon-based applications exist. For example, in radio frequency identification bands, patch antennas may be used at about 900 MHz; at about 13 MHz, coil antennas may be used. In order to build electronic circuits from organic semiconductor components, the problem of poor performance of organic semiconductor devices must be overcome.
Conjugated organic materials are organic materials where the electrons are crowded together near double or triple bonds. Conjugated organic materials are often treated as semiconductors with very low doping concentrations. Therefore, a rigid energy band structure at the interface between the metal and organic layers is often adopted. Due to the lack of surface states, the nature of the metal-organic interface, including barrier height and charge injection efficiency, is quite sensitive to the work function of the contact metal. These phenomena differ significantly from those of inorganic semiconductor where the mid-gap surface energy states, caused by the dangling bonds, pin the surface Fermi level. Hence, the silicon-metal interface weakly reflects the difference of the metal work functions. But, for organic diodes, the current rectification typically arises out of the difference in the work functions of the anode and cathode. For efficient charge injection and high rectification ratio, organic diodes require the use of high work function metals, such as gold or indium-tin oxide, as the anode, and low work function metals, such as calcium, as the cathode. However, despite the rapid progress in the field of organic light-emitting diodes and organic electroluminescence displays in the past ten years, the performance of general organic semiconductor devices such as diodes and transistors is still poor compared to inorganic semiconductor devices. As mentioned above, this is mainly due to the low carrier mobility, poor contact or junction between the metal and the organic material, and other characteristics of organic materials, which limit the applications of organic electronic devices to other areas, such as high speed wireless identification tags with predominantly or all organic components.
Further, in order to create a high performance organic transistor, high quality ohmic contacts between the electrodes and the organic material are generally required for efficient charge injection. However, current organic electronic devices lack a good ohmic contact and the prior art does not have a controllable process for creating an ohmic contact. In fact, one of the technological bottlenecks for creating high performance organic transistors will be the formation of good source and drain ohmic contacts to the organic layer. For certain types of devices, it is important to create a high quality rectified junction, such as a Schottky barrier junction or a p-n junction.
Therefore, there is a need for high performance organic semiconductor devices and a method for making them. Such high performance devices should have improved operating characteristics including, for example, better contacts and junctions between the metal and organic materials, the ability to operate at higher frequencies than presently possible, and/or higher current capacities.
One embodiment of the novel process creates a low resistance contact between a metal material and an organic material of an organic semiconductor device, which improves the efficiency of carrier injection, enhances operating characteristics of the organic semiconductor device such as operating speeds and current carrying capacity, and/or may create an ohmic contact and/or a Schottky barrier junction. Additionally, the process may cause metal ions or atoms to migrate or diffuse into the organic material, cause the organic material to crystallize, or both. The novel process may subject the organic semiconductor device to thermal or other forms of energy, such as voltage, current, electromagnetic radiation energy (e.g., laser energy) for localized heating, infrared energy and/or ultraviolet energy. An example result of the novel process is an enhanced organic diode comprising aluminum, carbon C60, and copper that can operate at high frequencies.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.